pvec: use the correct name for an identifier

Found using Cython.

mercurial/pvec.py

@@ -169,7 +169,7 b' class pvec(object):'
             self._bs = hashorctx
             self._depth, self._vec = _split(base85.b85decode(hashorctx))
         else:
-            self._vec = ctxpvec(ctx)
+            self._vec = ctxpvec(hashorctx)
 
     def __str__(self):
         return self._bs